Nick Clegg promised to rebuild the government's shattered relationship with the rest of Europe and risked opening a coalition rift by going public with his "bitter disappointment" at David Cameron's decision to block a new EU agreement.
The deputy prime minister said Britain risked becoming "isolated and marginalised" from the European mainstream and, along with senior Liberal Democrats, spent the weekend contacting European leaders in a "strategy for re-engagement to recover lost ground", according to a senior government source.
Several high-profile figures, including the former leader Paddy Ashdown and the party president, Tim Farron, joined Clegg in a wide-ranging attack on Cameron's resort to a British veto.
Clegg will hold a meeting with business leaders this week to convince them "they had not completely had the door shut", according to an aide. There is growing concern that the 26 EU countries who agreed on greater fiscal integration last week will now be able to strike deals affecting British banks and businesses.
The business secretary, Vince Cable, who warned the prime minister in Cabinet last Monday against the strategy he went on to follow in Brussels, is concerned that global companies including banks and pension funds will now shun investments in the UK, having previously favoured it as a "gateway" to the continent.
Clegg was biting in his critique of developments in Brussels but spoke of correcting the path chosen by Cameron by getting "back into the saddle". "I'm bitterly disappointed by the outcome of last week's summit, precisely because I think now there is a danger that the UK will be isolated and marginalised within the European Union," he told the BBC's Andrew Marr Show.
Describing his reaction when Cameron phoned him at 4am on Friday with news of the veto, the Lib Dem leader said: "I said this was bad for Britain. I made it clear that it was untenable for me to welcome it."
Tense discussions took place within and between the coalition parties, focusing on the slim chance that Britain could be back "in the room", if not actually involved, in the continuing talks on the deal. Clegg indicated he would be working in this area, hardening his rhetoric from the conciliatory tone in the immediate aftermath of the deal. Echoing the former Labour leader Hugh Gaitskell, he said: "I will fight, fight and fight again for Britain's long-term interests to make sure they are enhanced and not undermined."
Clegg said he would do "everything I can to ensure this setback does not become a permanent divide".
"As more summits and meetings occur, there will be other opportunities ahead to seek to exercise some influence."
The foreign secretary, William Hague, took issue with Clegg's description, telling Sky News: "We are not marginalised, I don't agree. I don't use the terminology 'two-speed Europe' – that implies there is one group getting on with something more quickly than another group." Senior Conservatives say the Lib Dem obsession with being "in the room" is facile.
No 10 sources were more conciliatory, saying the Tories would give the Lib Dems a chance to express their dismay and allow Clegg some room to deal with backbench and activist anger. A spokesman for Cameron suggested Clegg's comments may have been addressed to the "Eurosceptic right of the Conservative party". He said: "His comments were a warning across the bows to them that this is not step one to withdrawal which also remains our position. However it's no secret there are differences of approach."
Clegg's aides say he spoke out because he had been alarmed by the language of Tories who believe the veto is a precursor to eventual withdrawal from Europe. The deputy prime minister said those welcoming the outcome of the summit were "spectacularly misguided".
"There's nothing bulldog about Britain hovering somewhere in the mid-Atlantic, not standing tall in Europe, not being taken seriously in Washington," he said, warning that the UK was "retreating further to the margins" of Europe.
Clegg said if he had been at the summit then "of course things would have been different". He added: "I'm not under the same constraints from my parliamentary party that clearly David Cameron is."
Lib Dems pleaded for the prime minister to avoid triumphalism in his statement to the Commons after a weekend of celebrations by Eurosceptic backbenchers. A Downing Street source said Cameron's address would be consistent with the calm tone of his comments so far.
According to one source, the pro-European justice minister Ken Clarke has discussed resigning over Cameron's stance. Clarke aides deny this. Another suggestion that Cable might also resign was also denied as "categorically untrue".
Speaking on Radio Nottingham, Clarke said: "I think it's a disappointing, very surprising, outcome. There will be a big statement made by the prime minister on Monday. I shall be sitting there listening, but I also will be discussing what we are going to do between now and then."
